Noble metal coated ceramic substrate for glass seals and electronic connector elements



NOBLE METAL COATED CERAMIC SUBSTRATE FOR GLASS SEALS July 2, 1968 s, J,SULLWAN ET AL 3,390,969

AND ELECTRONIC CONNECTOR ELEMENTS Filed April 27, 1966 ZSheets-Sheet 1'July 2, 1968 s. .1. SULLIVAN ET AL 3,390,969

NOBLE METAL COATED CERAMIC SUBSTRATE FOR GLASS SEALS AND ELECTRONICCONNECTOR ELEMENTS Filed April 27, 1966 2 Sheets-Sheet 2 Z, 70 Q5 22 i//'a 6 9 lf L Inc/W United States Patent 3,390,969 NOBLE METAL COATEDCERAMIC SUBSTRATE FOR GLASS SEALS AND ELECTRONIC CONNEC- TOR ELEMENTSStephen J. Suliivan, Weston, and Merritt W. Albrlght, West Peabody,Mass., assignors to Infrared Industries, Inc., Santa Barbara, Calif., acorporation of Delaware Continuation-impart of application Ser. No.333,476, Dec. 26, 1963. This application Apr. 27, 1966. Ser. No. 560,392

Claims. (Cl. 29-195) ABSTRACT OF THE DISCLOSURE This invention relatesto a laminated material wherein a ceramic substrate is coated insequence with a metal oxide, a metal selected from the group of copper,nickel, chromium, manganese, titanium, stainless steel, andnickelchromium, and a noble metal selected from the group consisting ofgold, silver, platinum, rhodium and iridium. The material is utilized inmaking glass to glass seals and for electronic components especiallyphotoconductor detectors.

This application is a continuation-in-part of application Ser. No.333,476 filed Dec. 26, 1963, now abandoned.

The present invention relates to metal coated substrates useful inmaking a joint, a seal, a weld or the like between two elements, and toits preparation. More particularly, this invention relates to a uniquepermanently bonded metallic film on a non-metallic substrate. The filmis particularly useful as a surface joint in a glass to glass seal suchas in glass tubular constructions employed in the instrumentation andelectronic fields. It is also particularly useful as a base forelectronic elements whereby metallic leads may be easily secured theretofor effecting a circuit with said element.

In the preparation of various electronic devices, and more particularlyin the preparation of electronic tubes, substantial evacuation of theinterior is frequently required after the various tube elements areplaced therein. In certain instances, such as in photoconductor cellswhere indium antimonide is employed, the glass to glass seal usuallyprovided to seal the elements within the tube, the seal is preferablymade at the lowest temperature possible. Otherwise, photoconductorelements may be damaged and possibly completely destroyed.

Sealing techniques have been employed in the past such as disclosed inUnited States Patent No. 2,446,277 issued Aug. 3, 1948, entitled Glassto Metal Seal in Electrical Devices. Such seals, however, do notcontemplate a glass to glass relationship where low temperature sealingis of paramount importance, and where the electrical property of theseal is of no major significance. In addition, the seals which haveheretofore been employed often merely rely upon a physical interlockingrelationship between the glass and metal.

In certain structures, such as multi-element infrared detector arrays,it is desirable to place the detector elements on a base, such as quartzand then affix leads thereto. Since the element is extremely fragile, itis difficult to do this. So, as an alternative, the trade fastens thedetector elements to a highly conductive metallic layer, such as gold,on an insulator base and the leads are then affixed to the metalliclayer. In prior art techniques, the metallic layer was only looselyaffixed to the base such as by metallic evaporation or by gold leafbonding or the like.

3,390,969 Patented July 2, 1968 ice Shifting and breakage of the layerfrequently occurred. The resultant malfunction was oftentimes extremelycostly due to the fact that successful operation of the detector elementtriggered the entire subsequent operation of the costly mechanism.

An object is to provide a novel metallic coated substrate structure as aresult of the technique.

Another object is to provide a bonded metallic layer on a substratewherein the bond is substantially integral so that separation isvirtually impossible.

Still another object is to provide a method for the preparation of aseal which can be accomplished even at very low temperatures in therange from F. to 350 F. A related object is to provide such a seal,which may be optionally made between two glass elements, or glass andmetal. Indeed, when desired, the same low temperature sealingcharacteristics can prove advantageous between two elements in a metalto metal seal.

Another object is to provide a seal susceptible of low temperaturecompletion which is sturdy, durable, and calculated to outlast the lifeof the device enclosed by the seal.

Another object is to provide a novel metallic coated substrate usefulfor bonding heat, light and similar detector elements thereto.

Another object is to provide a novel metallic coated substrate, of theabove character, in miniature size but to highly accuratespecifications.

Further objects and advantages of the present invention will becomeapparent as a description of several embodiments illustrative of theinvention proceeds.

FIG. 1 is a front elevation of an indium antimonide detector employing aglass to glass seal illustrative of the present invention.

FIG. 2 is an enlarged, exploded, cutaway, partially diagrammatic view ofthe two tubular elements which are sealed in a manner illustrative ofthe invention.

FIG. 3 is an enlarged diagrammatic, cutaway transverse section of an endof a tubular member prepared for a solder seal in accordance with amethod illustrative of the subject invention.

FIG. 4 is a diagrammatic view of an evaporating chamber illustrating howthe surfaces of the tubular members are prepared for sealing.

FIG. 5 is an exploded perspective view of the detector shown in FIG. 1illustrating the relationship between the solder ring and the twotubular elements to be sealed.

FIG. 6 is a top plan view of a substrate upon which a metallic film isto be bonded.

FIG. 7 is a side cross-sectional view of the metallizing apparatus forsuch bonding.

FIG. 8 is a top plan view of the finished substrate having an array ofinfrared detector elements afiixed thereto in circuit arrangement.

FIG. 9 is a side cross-sectional view of the finished substrate andready for use as an infrared detector.

In broad outline, the subject invention contemplates a method for thepreparation of a metallic coated substrate wherein the bond is so strongthat the structure is useful as part of a joint for sealing or as a basefor mounting fragile or delicate electronics elements thereto in circuitarrangement. The invent-ion also embraces the joint and the finisheddetector element.

In forming a joint, the elements to be sealed are first provided withcomplementary faces, normally the two faces of the end of a glass tube.The faces of the host material are chemically cleaned, and heated todrive out occluded water and volatile impurities prior to placing thesame in an evaporating chamber. A base metal is first evaporated ontothe faces which are to be sealed in the presence of sufficient oxygen toprovide a base metal oxide in an absorptive soluble relationship withthe end faces of the tubes. Thereafter the base metal is evaporated soas to form a thin layer atop the oxide layer and in a mutuallyabsorptive or soluble relationship with the oxide. Then, and preferablyprior to concluding the evaporation of the base metal, evaporation of anoble metal is instituted so as to'form a layer of the noble metal atopthe base metal. With such overlapping concurrence of evaporation, adiffused interface of noble metal and base metal results.

The same technique is used for forming the metal coated substrate. Thesubstrate, usually of a dielectric material such as quartz, ceramic,aluminum oxide, plastic, or glass is chemically cleaned, then heated todrive off occluded water. It is then placed in an evaporating chamberand appropriately masked to effect a desired circuit. Then an alloy,such as gold alloy, followed by the pure metal, such as gold, isevaporated onto the unmasked portions of the substrate. This sequence ofalloy-pure metal evaporation results in a diffused interface of puremetal-metal alloy-substrate. It appears that the mechanism of theexcellent bond of the base metal to the quartz substrate is apparentlycreated by a chemical bond. The intense afiinity for oxygen of the basemetal combines with a free valence of the oxygen in the substrate,silicon oxide in the case of quartz, to form a strong chemical bond. Theaffinity of this bond is further metered by specific percentages ofother base metals allowing fabrication peculiar to specific circuitry,such as infrared detector manufacture. In the latter, the bond adhesionmust be able to withstand chemical cleaning, such as boiling in nitricacid and yet such that it may be diamond scribed without cutting deeplyinto the quartz base. The technique of this invention accomp'lishessatisfactory bond adhesion.

For the purposes of a more detailed description of the subjectinvention, absorptive relationship refers to that relationship betweenthe metal or oxide being evaporated and its adjacent surface which formsan intimate bond either by way of absorption, molecular interlacing, orsolubility in a degree not fully or clearly understood or definedherein, but observable and detectable to that extent necessary topractice the subject invention and achieve good results. The inventionis not limited to the surface phenomenon, but to the steps employed andthe results achieved. The base metal is selected from that group ofmetals which are readily evaporable in known evaporation chambers, andwhich, in the presence of small quantities of oxygen in the course ofsuch evaporation, will form an oxide which finds itself in readyabsorptive relationship with the host glass, ceramic, quartz, plastic,or metal, such base metals including aluminum, chromium, copper,manganese, nickel, nickel-chromium alloys (Nichrome), titanium andvarious stainless steels. The noble metal which is finally evaporatedatop the base metal includes gold, silver, platinum, rhodium, andiridium. It is also possible, in certain specific instances, to utilizea non-noble metal, such as aluminum, lead and copper.

The invention will be better understood by reference to descriptions ofthe actual preparation of a seal during the manufacture of a detector 10of the character illustrated in FIG. 1 or of an infrared detectorassembly illustrated in FIG; 8.

In FIG. 1, it will be observed that the outer shell of the detector 10comprises a glass envelope or tube 11, which in turn consists of anupper tube 12 and a lower tube 14. The glass tube 11 is joined to ametal base 15, and houses a detector element 16 in its interior portion.

The joint is illustrated in accordance with its preparationdiagrammatically in FIG. 2. As shown, the upper tube 12 and the lowertube 14 are provided with complementary faces, indicated as upper tubeend 18 and lower tube end 19. While the joint faces have been shown hereas the cutolf end of a tubular member, it will be appreciated thatelliptical configurations, not necessarily lying in a single plane, mayalso be sealed in accordance with the method described herein.

As will become apparent during the description of the method, thepreparation of the face of the tube body 22 with a tube end coating 21,such as illustrated in FIG. 3, is the heart of the invention. From FIG.3 it will be observed that the tube end 22 has atop it, in an absorptiverelationship, an oxide layer 24 of approximately molecular thickness.Thereafter, the oxide layer is covered by the base metal 25 to a depthof about .075 to .10 micron. Prior to the completion of the depositionof the base metal layer 25, evaporation is started of the noble metal28, and because of the contemporaneous overlapping period ofevaporation, a blended interface 26 in which the base metal moleculesand noble metal molecules are inextricably interwoven is defined. Thenoble metal overlay or outer coating 28 is of a thickness from .075 to.10 micron ideally, but if the layer is somewhat thicker, the seal willstill be satisfactory.

Turning now to FIG. 5, it will be seen that after the tube end coating21 has been applied in accordance with the above descriptive materialrelating to FIG. 3, the upper tube 12 and lower tube 14 may be joined bymeans of a solder ring 36 which is placed between the two tube portions12, 14, and the two portions are brought together in such a manner as tophysically hold the solder ring 36 therebetween, and thereafter by meansmost readily adaptable to the assembly techniques, the adjacent areasurrounding the solder ring 36 is heated to a sufficient temperature tomelt the exterior portion of the solder ring 36 and form a bond with thetube end coating 21, and more particularly the outer noble metal portion28 thereof. Subsequently, where required, an evacuation tube 38 such asillustrated may be employed to evacuate the interior portion of thetube, sealing the evacuation tube 38 in accordance with knowntechniques.

In preparing the tubular portions 12, 14 for the joint, when glass isemployed it is chemically cleaned by dipping it in a hot chromic acidsolution followed by a distilled water wash. In this manner anyextraneous chemicals, greases, and other foreign materials are removedand a clean surface results. Thereafter, the glass or metal is heated toat least 500 C., or any temperature sufiiciently short of the softeningof the material in order to expel any occluded gases and water vapor.

The tubular members 12, 14 are then placed inside an evaporating chamber29 such as illustrated in FIG. 4. Each is positioned from a convenienttube support diagrammatically illustrated, but the tube support 30 maybe of varying configurations depending upon the glass elements to betreated. Subsequently, the chamber is evacuated to 10- mm. of mercury orany other compatible degree of evacuation with reference to theevaporation distance between the filaments and their metal forevaporation in the face to be plated.

As will be observed, a base metal filament 31, and a noble metalfilament 34 are provided within the evaporation chamber 29. A base metalribbon 32 (or base metal powder) is applied to the base metal filament31. Similarly, a noble metal ribbon 35 (or powder) is applied to thenoble metal filament. After the chamber is suitably evacuated to removeall but small contamination proportions of oxygen, the base metalfilament is first heated for a period of about a minute. This results inthe deposition of the first layer of oxide 24, followed by the .075 to.10 micron layer of base metal 25, preferably chrominum oxide andchromium respectively. Prior to turning off the current or heat sourceto the base metal filament 31, the noble metal filament 34 is heated inorder to commence the evaporation of the silver ribbon 35 to define theblended interface 26.

After the silver or noble metal has been deposited in a sufficientlythick layer, the filaments are turned off, and the treated tubularelements 12, 14 removed, and a nonfluxing solder ring 36 is appliedthereto and the tube assembled as illustrated in FIG. 5 and describedabove.

Example I Two Pyrex brand glass tubes were ground for a, complementaryfit between their two ends. The ends were then cleaned by dipping themin a chromic acid bath at a temperature of 25 C. After washing withwater, the tubes were dried in an oven heated to a temperature of 150for a period of 60 minutes. Such temperature treatment completely driedthe glass tubes and removed any absorbed gases.

The tubes were then placed, end up, within an evaporation chambermanufactured by Vacuum Specialties of Sommerville, Mass. Within such achamber was positioned a base metal filament 31, as illustrated in thedrawing, consisting of tungsten, and a noble metal filament 34consisting of tungsten. A ribbon of chromium and silver, respectively,were straddled upon these filaments. The chamber was then evacuated tomm. of mercury. Both the noble metal and base metal filaments werepreheated to a point slightly below the evaporative level. The evaporantwas then elevated to a temperature of 1205 C. for a period of twominutes by applying 3, voltage of 70 volts, at which time boiling may beobserved. This resulted in deposition of a base meal oxide layer 24, asseen in the drawing, followed by a base metal layer 25. The two layersapproximate 2 microns in thickness. About 30 seconds before terminatingthe evaporation of the base metal a current of 50 volts was supplied tothe noble metal filament 34. The noble metal evaporant rose to atemperature of approximately 1047 C., and this caused evaporation of thesilver ribbon with consequent formation of a blended interface 26 uponeach end of the two glass tubes. The noble metal current was suppliedfor an additional two minutes.

After complete deposition, the current to filament 34 was terminated,the chamber was bled to atmospheric pressure, and the tubes 12 and 14removed. For assembly, a non-fluxing solder ring 36 comprising 60% tin,40% lead, was interposed between the two ends of the tubes. This ringwas heated by radiant ring heater, to a temperature of 188 C. to causethe outer layers of solder ring to fuse to the ends of the tubes. Anintegral seal is formed.

During testing, the seal was found to be positive. There was no leakageand a very dependable seal had evidently been created between the twotubes.

The temperatures employed for vaporization of the base and noble metalsvaries with their vaporization point and the pressure utilized in theevaporation chamber. Generally temperatures of 1000 C. to 1500 C. andpressures of 5 l0 to 1 10 mm. mercury are used. The foregoing voltageswere established empirically to achieve the evaporation required.

While the description of a joint formation herein is related primarilyto the employment of chromium as the base metal, and silver as the noblemetal principally because of their ready availability, relatively modestcost, and superior qualities, both from a standpoint of method andultimate seal, alternative metals may be employed in forming a joint. Asstated above, alternate base metals include copper, nickel,nickelchromium alloys and various stainless steels. The alternate noblemetals which may be used are platinum, rhodium, iridium, and gold.

Referring now to FIG. 6 for a detailed description of the metal coatedsubstrate, as used for an electronic component, and technique for itspreparation, there is shown the raw fiat substrate 61 upon whichmetallic fil-m 62, in this particular instance, gold, is to be bonded ina desired configuration. The configuration shown in FIG. 8 is useful formaking a multi-element detector array hereinafter.

The finished product is shown in FIGS. 8 and 9. As seen therein, itcomprises the substrate 61 with gold film adhered thereto in asubstantially permanent manner.

The gold film pattern is formed by the use of an evaporation mask.Initially, gold is deposited in strips 63 to 71 joined by a commonheader 72. Detector elements 73 to 76 overlie arms 64, 66, 68 and 70, asseen in FIG. 9. A second layer of metal is deposited over a portion ofsaid arms and said detector elements to form an intermeshingrelationship for better electrical communication and to accurately maskeach element to expose only a predetermined accurate area thereof. Thusmulti-element detector arrays, which are not only uniform, but whichhave reproducible results from array to other array, can be manufacturedrather easily.

The following example illustrates the detailed manufacturing stepsinvolved.

Example II A plurality of quartz substrates 61 having a dimension 0.5 x1 x 0.027 inch are chemically cleaned by dipping in hot chromic acidfollowed by washing in pure water, and then heating to a red heat,followed by cooling. The substrates are then masked with an appropriatemask to effect the pattern desired, such as seen in FIG. 8 and thenmounted face down in the holding plate 77, preferably using plastictipped tweezers to avoid contamination. A thermocouple 78 is placedagainst the back end of each substrate for accurate measurement of thetemperature of the substrate as the process proceeds. Each substrate inholding plate 76 is then positioned beneath heater 79. The temperatureof the substrate is raised about 200 C. 5 C. Actually, a range of 150 to350 C. is operable.

As shown, the exposed face of each individual substrate 61 faces theinterior of a vacuum metallizing chamber 80. Within the chamber are apair of filament boats 81 and 82.

Boat 81 contains a gold alloy of the following composition:

Percent Gold 72 Nickel 22 Chromium 6 Tolerances 10.5

Boat 82 contains pure gold. The boats are successively heated, the alloyboat first.

The vacuum metallizing chamber is flushed with an inert gas, such asnitrogen, helium or argon, and then reduced to a vacuum of 0.5 l0microns pressure. Actually, the pressure may range from 1 10- to 1X l0microns.

Then the individual boats are successively heated, the alloy boat first,generally between 2000 to 5000 C., by a heating means, such as atungsten filament (not shown) to vaporize the charge of each boat. Thetotal evaporation time is about six and one-half hours. A film having athickness of about 5000 Angstroms, although the range of thickness mayvary between 2000 to 20,000 Angstroms, deposits onto the exposed face ofthe substrate. A cross section through the film will reveal a gold alloylayer adjacent the substrate, a gold alloy-pure gold interface layer anda top layer of pure gold, much like that shown in FIG. 3. The top layer,in this instance, is pure gold, rather than silver, for betterelectrical conductivity.

Strips having a width as little as .002 inch on a centerline betweeneach of .005 inch may be effected. It should be evident that such minutestructure enables attachment of leads having a dimension no greater than.0005 inch. Thus, miniaturization is possible.

After the evaporation, the substrate is allowed to cool to C. or less,and then the vacuum system is bled with an inert gas followed by air. Itis then removed and the evaporation mask taken off the substrate. It isthen stored in a plastic box until ready for application of the detectorelements, lead sulfide, (PbS) in this instance, and attachment of leads.

The PbS is applied as a coating in the uncoated portion (see FIG. 9) andadjacent coated portion of arms 64, 66, 68 and 70.

Then a second coat of gold is applied over the edge portions of PbS (see78 and 79 of FIG. 9) and across arms 65, 67, 69 and 71 (see 80 of FIG.9) to complete a series of circuits (see FIG. 8).

Leads 83 to 91, in this instance gold wire .0005 inch in diameter, arespot welded to the arms 63 to 71 and then the total array capped with atransparent cover 92. The detector array is now ready for incorporationinto a circuit of desired function.

To illustrate the strength of the bond between the metal film and thesubstrate, the following three tests are performed:

Scotch tape test. Thermocompression bonding test. Scribing test.

In a successful Scotch tape test, no more than of the film must pullotf. In the thermocompression bonding test, the breaking strength isnoted. Each bond shall have a breaking strength of not less than 12 gms.per lead (24 gms. for loop of two bonds). Qualification acceptance ofthe evaporation shall be satisfied after ten consecutive samples havepassed the breaking strength requirements. The scribing test consists ofthe number of passes required by diamond scribing to isolate conductors.One piece from each evaporation shall be scribed. The maximum number ofpasses needed to isolate shall not exceed 25. If more than 25 passes arerequired for conductor isolation, then the entire evaporation batchshall be rejected. The criterion for each satisfactory scribe shall bethat the scribed area be visually clear with 100x scope with bottomlight.

Substantially all substrate made met the above conditionssatisfactorily.

By using gold as the noble metal where a coating may be placed on flatglass (or other material), leads may be welded to the gold noble metallayer with an adhesion of unusual strength. For example, .0005 inch wireleads have been so welded with a Kulicke and Sofia thermocomperssionwelding machine to gold lines of .001 inch scribed in a glass plate.These lines may be either scribed from a solid evaporated layer orformed by evaporation through a mask.

Although particular embodiments of the invention have been shown anddescribed in full here, there is no intention to thereby limit theinvention to the details of such embodiments. On the contrary, theintention is to cover all modifications, alternatives, embodiments,usages and equivalents of the surface joint and preparation thereof asfall within the spirit and scope of the invention, specification and theappended claims.

We claim:

1. A surface joint for a ceramic insulating material comprising insequential order the ceramic insulating material, a base metal oxidelayer bonded to the host material and in absorptive relation therewith,a base metal layer atop the base metal oxide and in absorptiverelationship therewith; said base metal being selected from the classconsisting of chromium, copper, nickel, nickelchromium alloys, andstainless steel; and a noble metal layer bonded to the base metaldefining therebetween a diffused interface of base and noble metal; saidnoble metal being selected from a class consisting of gold, platinum,rhodium, iridium and silver.

2. A glass to glass seal comprising a pair of glass elements havingcomplementary adjacent faces, a base metal oxide layer bonded to eachglass face, a base metal layer bonded to the base metal oxide layer,said base metal being selected from the class consisting of chromium,copper, nickel, nickel-chromium alloys and stainless steel, said oxidebeing that of its adjacent base metal; a noble metal layer bonded to thebase metal layer and defining therebetween a diffused interface of baseandnoble metal; said noble metal being selected from a class consistingof gold, platinum, rhodium, iridium and silver; and a solder layer insealed soldered relationship with each noble metal layer thereby joiningthe two glass elements in sealed relationship.

3. A glass to glass seal comprising a pair of glass elements havingcomplementary adjacent faces, a chromium oxide layer adjacent each glassface, a chromium metal layer atop the chromium oxide layer, a silverlayer atop the base metal layer and defining therebetween a diffusedinterface of chromium and silver, and a solder layer in sealed solderedrelationship with each silver layer thereby joining the two glasselements in sealed relationship.

4. A metal coated substrate comprising, in sequential order, a ceramicinsulating material, a base metal oxide layer bonded to said ceramicinsulating material and in absorptive relation therewith, a base metallayer bonded to the base metal oxide and in absorptive relationshiptherewith, said base metal being selected from the class consisting ofchromium, copper, nickel, nickel-chromium alloys, titanium, andstainless steel, and a noble metal layer bonded to the base metaldefining therebetween a diffused interface of base and noble metal, saidnoble metal being selected from the class consisting of gold, silver,platinum, rhodium, and iridium.

5. The metal coated substrate of claim 4 wherein said ceramic insulatingmaterial is selected from the group consisting of quartz, ceramic,aluminum oxide, plastic and glass.

6. A glass to glass seal comprising a pair of glass elements havingcomplementary adjacent faces, a chromium oxide layer adjacent each glassface, a chromium metal layer bonded to the chromium oxide layer, asilver layer bonded to the base metal layer and defining therebetween aditfused interface of chromium and silver, and a solder layer in sealedsoldered relationship with each silver layer thereby joining the twoglass elements in sealed relationship.

7. An electronic element comprising a composite, said compositecomprising a ceramic insulating material, a base metal oxide layerbonded to the host material and in absorptive relation therewith, a basemetal layer bonded to the base metal oxide and in absorptiverelationship therewith; said base metal being selected from the classconsisting of chromium, copper, nickel, nickel-chromium alloys, andstainless steel; a noble metal layer bonded to the base metal definingtherebetween a diffused interface of base and noble metal; said noblemetal being selected from a class consisting of gold, platinum, rhodium,iridium and silver, a detector secured to said noble metal; and a leadafiixed to and extending from another face of said noble metal.

8. The electronic element of claim 7 wherein said layers of materialbonded to ceramic insulating material is divided into a plurality offinely divided strips so as to form a multi-element detector array witha plurality of detectors, one secured to each of the noble metalsurfaces of said strips and with leads affixed to and extending fromeach strip.

9. The electronic component of claim 8 wherein said detectors comprisePbS.

10. An electronic element comprising in sequential order a ceramicinsulating material, a base metal oxide layer bonded to the ceramicinsulating material and in absorptive relation therewith, a base metallayer bonded to the base metal oxide layer and in absorptive relationtherewith, said base metal being selected from the class consisting ofchromium, copper, nickel, nickel-chromium alloys, and stainless steel, anoble metal layer bonded to the base metal layer and definingtherebetween a diffused interface of base and noble metal, said noblemetal being selected from the class consisting of gold, platinum,rhodium, iridium, and silver, said ceramic insulating mate- 9 rialhaving portions of the coating thereon removed 50 3,106,489 as to formseparate areas of the coating, and leads con- 3,107,756 meeting to thecoating in the separate areas. 3,115,957 3,197,290 References Cited 53,243,313

UNITED STATES PATENTS 2,975,078 3/1961 Rayfield 117-217 3,098,930 7/1963Clark 136-214X 10 Lepselter. Gallet. Heil 29195 X Williams 29195 Aves.

HYLAND BIZOT, Primary Examiner.

